This research is concerned with factors that influence suckling and its neural management during ontogeny. This research will describe, identify, and/or analyze (1) the sensory events that determine nipple-attachment, (2) the internal stimuli that determine milk intake at the nipple, (3) the neural and neurochemical substrates that support self-stimulation in newborn rats, and (4) the spontaneous interactions between mother rats and their young and mother opossums, Monodelphis domesticus, and their young under semi-natural conditions. This last aim will assess the generality of our findings to suckling as it may appear naturally.